Warm rolling of steels containing metastable austenite

ABSTRACT

Warming a metastable steel before or during cold rolling suppresses the transformation of austenite to martensite, resulting in lower mill loads and higher amounts of reduction at similar loads. As-warm rolled steel has enhanced mechanical properties when compared to steel reduced the same amount by cold rolling. Warm rolling followed by subsequent annealing also results in better mechanical properties than those achieved in material cold rolled the same amount and then annealed. Metastable steel that has been warm rolled, on subsequent room temperature rolling (cold rolling), also shows enhancement in both strength and ductility

PRIORITY

This application claims priority to U.S. Provisional Application Ser. No. 62/278,567, entitled WARM ROLLING OF STEELS CONTAINING METASTABLE AUSTENITE, filed on Jan. 14, 2016, and U.S. Provisional Application Ser. No. 62/407,001, entitled WARM ROLLING OF STEELS CONTAINING METASTABLE AUSTENITE, filed on Oct. 12, 2016, the disclosures of which are incorporated by reference herein.

BACKGROUND

Cold rolling of steels containing metastable austenite can be challenging due to deformation induced transformation of metastable austenite to a higher strength martensite phase. Cold rolling of such steel leads to a significant increase in mill loads. Steel also needs to undergo annealing(s) to partial or full austenitization before further cold reduction can be performed.

SUMMARY

The present invention involves warming the material before or during cold rolling to suppress the transformation of austenite to martensite. This can result in lower mill loads and higher amounts of reduction at similar loads. The ability to reduce material more can also lead to fewer intermediate anneals before material can get to final gauge. Surprisingly, as-warm rolled steel has shown enhanced mechanical properties when compared to steel reduced the same amount by cold rolling. Warm rolling followed by subsequent annealing also results in better mechanical properties than those achieved in material cold rolled the same amount and then annealed. Steel that has been warm rolled, on subsequent room temperature rolling (cold rolling), shows enhancement in both strength and ductility.

Previously, warm rolling has been avoided in the production environment because of concern that it may cause damage to rolling equipment as well as present risks related to warming of the oils used as lubricants. The present application shows that the benefits of warm rolling can be achieved at moderate temperatures and without extensive line modifications.

DESCRIPTION OF FIGURES

FIG. 1 depicts percent martensite in a metastable steel as a function of percent reduction resulting from warm rolling and cold rolling.

FIG. 2 depicts percent elongation in a metastable steel as a function of percent reduction resulting from cold rolling and warm rolling.

FIG. 3(a) depicts true stress-true strain curves for a metastable steel that was warm rolled and then cold rolled.

FIG. 3(b) depicts true stress -true strain curves for a metastable steel that was cold rolled in two passes.

DETAILED DESCRIPTION

This invention pertains to steels containing significant amount of metastable austenite (10%-100% austenite), referred to as “metastable steel.” Austenite is deemed metastable if it transforms to martensite upon mechanical deformation. Such martensite is called deformation-induced martensite. Steels containing such metastable austenite can be carbon steel or stainless steel.

There are several ways to characterize the stability of austenite. One way is to calculate an Instability Factor (IF) of the austenite based on its chemical composition. This factor was described in U.S. Pat. No. 3,599,320 (the disclosure of which is incorporated herein by reference), which defines IF as:

IF=37.193-51.248(% C)−0.4677(% Cr)−1.0174(% Mn)−34.396 (% N)−2.5884(% Ni)   Equation 1

Steels with calculated IF values from 0-2.9 are categorized as “slightly metastable” and steels with IF greater than 2.9 are categorized as “moderately metastable”. The method of the present invention has the most significance for steel containing metastable austenite with an IF greater than 2.9.

Another technique to characterize the stability of austenite is to calculate or measure what is known as the M_(d)30 temperature. For a given metastable steel composition, on deformation to 0.3 true strain at the M_(d)30 temperature, 50% of the austenite transforms to martensite. For a given metastable steel composition, the M_(d) temperature is the temperature above which no martensite is formed upon deformation. M_(d) and M_(d)30 temperatures are well-known in the art. In addition to being empirically determined, the M_(d)30 temperature for a particular steel composition can also be calculated by one of the several equations that can be found in literature, including the following:

As taught by Nohara, K., Ono, Y. and Ohashi, N. 1977. Composition and Grain-Size Dependencies of Strain-Induced Martensitic Transformation in Metastable Austenitic Stainless Steels. Journal of Iron and Steel Institute of Japan, 63 (5), pp. 212-222 (the disclosure of which is incorporated herein by reference):

M _(d)30=551−462(% C+% N)−68*% Cb−13.7*Cr−29(% Cu+% Ni)−8.1*% Mn−18.5*% Mo−9.2*% Si   Equation 2

As taught by Angel, T. 1954. Formation of Martensite in Austenitic Stainless Steels. Journal of the Iron and Steel Institute, 177 (5), pp. 165-174 (the disclosure of which is incorporated herein by reference):

M _(d)30=413-462*(% C+% N)−13.7*% Cr−8.1*% Mn−18.5*% Mo−9.5*% Ni−9.2*% Si   Equation 3

The higher is the M_(d)30 temperature of the austenite of a given metastable steel composition, the more unstable is the austenite. M_(d)30 temperature in such metastable austenite is above the M_(s) temperature (martensite start temperature of thermal martensite).

Steels with a significant amount of metastable austenite work hardens rapidly as the austenite transforms to higher strength martensite. This work hardening, and resulting martensite, can present a challenge when further cold rolling such steels because they can require loads that may exceed a mill's capability. Such metastable steels then need to be annealed to form some or all austenite before they can be cold rolled further. If during rolling, transformation of austenite to martensite can be suppressed, the steel can be rolled to thinner gauges, with lower mill loads. One way to suppress such transformation is to warm the material prior to or during cold rolling. Warm rolling has shown to have additional benefit resulting in better mechanical properties.

The methods of the present application involve rolling such metastable steels while the steel is warm. It is considered warm when the metastable steel temperature is above room temperature (typically about 80° F.). For certain embodiments, the steel is warmed to a temperature near or above the M_(d) temperature for the particular metastable steel composition. In other embodiments, the steel is warmed to a temperature above the M_(d)30 temperature for the particular metastable steel composition. In other embodiments, the metastable steel is warmed to a temperature less than or equal to 250° F.

The coils of such material can be warmed in ways that will be apparent to one of skill in the art, including one of or a combination of the following methods:

I. Warm the coil in a furnace/oven prior to putting it on the rolling line.

II. Warm the coil on the line by using heaters, before it is cold rolled.

III. Warm the coolant on the mill before rolling the steel material. This can be performed in several ways. One way is to turn off the cooling tower on rolling mill and run some other material to warm up the coolant prior to rolling the metastable steel. Other methods of warming the coolant prior to rolling will be apparent to those of skill in the art.

The metastable steel is melt, cast, hot rolled, and annealed prior to cold rolling (if applicable) in accordance with typical metal-making processing for the particular composition. During the cold rolling processing of the metastable steel, at least one “cold rolling” pass is a “warm rolling” pass that is performed while the steel is warm, i.e., while the steel is at a temperature above 80° F. In some embodiments, the steel is warmed to a temperature no greater than 250° F. In other embodiments, the metastable steel is warmed to a temperature near or above the M_(d) temperature for the particular metastable steel composition. And in other embodiments, the metastable steel is warmed to a temperature near or above the M_(d)30 temperature for the particular metastable steel composition. Such warm rolling passes can be one or more of the first, second, or any subsequent “cold rolling” steps.

In some embodiments of the present invention, the metastable steel may be annealed after one or more warm rolling step. For example, during the “cold rolling” processing, the metastable steel may be warm rolled in a first pass, annealed, and then cold rolled (at room temperature) in a second pass.

Example 1

A metastable steel was prepared by melting a heat with a chemistry that had an Instability Factor of 8.5 and M_(d)30 (Nohara)=447.6° F. The heat was continuously cast into slabs. The slabs were re-heated to 2300° F. and hot rolled to a thickness of 0.175″, with a coiling temperature of 1000° F. The hot band was the then pickled to remove the scale. Sections of the pickled hot bands were cold rolled and warm rolled. For purpose of warm rolling, the hot band sections were warmed to desired temperatures in a furnace and rolled to desired gauges.

FIG. 1 shows the amount of martensite transformation from cold and warm rolling of such metastable steel. At the same amount of reduction, the amount of martensite in each warm rolled steel is significantly less than in cold rolled steel, which was rolled at room temperature. The benefits of warm rolling can be seen at low temperatures (150° F. in this example) but the higher the temperature during warm rolling (250° F. in this example), the lower is the amount of martensite formed.

FIG. 2 shows the % elongation of the metastable steel, after warm rolling and cold rolling to different reduction amounts. Surprisingly, warm rolling led to an increase in % elongation till certain amount of reduction before starting to drop. The benefits of warm rolling can be tailored by either varying the amount of reduction performed at a temperature or by varying the temperature. On the other hand, cold rolling at room temperature always leads to a decrease in % elongation for metastable steels.

Example 2

Another metastable steel was prepared by selecting a chemistry with an Instability Factor of 13.11 and M_(d)30 (Nohara)=227.6 ° F. The heat was cast into ingots. After trimming the ingots, four bars of 5.75″ (W)×2.75″ (T)×2.75″ (L) were obtained. These trimmed ingots were soaked at 2200° F. and hot rolled to 0.2″ with a finishing temperature of 1100° F. The hot band was then pickled to remove the scale. Sections of the pickled hot bands were cold rolled and warm rolled at different temperatures. For purposes of warm rolling, the hot band sections were warmed to the desired temperatures in a furnace and rolled to desired gauges.

In such metastable steel, warm rolling followed by cold rolling showed an increase in both strength and % elongation. Without prior warm rolling, the same steel showed an increase in strength but a decrease in % elongation, as expected. FIG. 3(a) shows true stress strain data from the metastable steel that had been warm rolled 30% and subsequently cold rolled at room temperature to various reductions. In FIGS. 3(a) and 3(b), “WR” refers to warm rolling and “RT” refers to cold rolling at room temperature. 30% warm rolling followed by additional 10% cold rolling showed an increase in both elongation and strength. The same material when cold rolled by 30% followed by an additional cold rolling at room temperature of 0-30%, as shown in FIG. 3(b), showed an increase in ultimate tensile strength (“UTS”) but a decrease in elongation, as one would expect. Again, the benefits of warm rolling can be tailored by either varying the amount of reduction performed at a temperature or by varying the temperature.

Example 3

The metastable steel of Example 1 above shows the effect of warm rolling on steel containing metastable austenite as further shown by the test data set forth in the Tables 1 and 2 below, which compares properties of the steel containing metastable austenite that has been fully annealed (Coil 1) with steel containing metastable austenite that was 25% warm rolled in the plant (Coil 2).

TABLE 1 Longitudinal Yield Strength Elong. @ 0.2% Offset (Manual in 2″) Hardness (MPa) UTS (MPa) (%) Coil 1 Coil 2 Coil 1 Coil 2 Coil 1 Coil 2 Coil 1 Coil 2 (HRB) (HRC) Avg. 386.2 1197.6 1142.4 1551.8 52.6 21.8 98 46.6 Min. 376.1 1173.7 1126.7 1561.6 48.8 18.9 Max 394 1221.5 1164.8 1540.2 57.9 24 Transverse Elong. 0.2% OYS (Manual in 2″) Hardness (MPa) UTS (MPa) (%) Coil 1 Coil 2 Coil 1 Coil 2 Coil 1 Coil 2 Coil 1 Coil 2 (HRB) (HRC) Avg 423.6 1111.9 1128.8 1520.8 54.5 19.6 98 47.3 Min. 394.6 1092.7 1104.5 1504.5 50.7 17 Max 435.7 1133.1 1149.3 1531.2 57.7 21.8

TABLE 2 Test Average (Coil 1) Average (Coil 2) Ultimate Tensile Strength 1188 MPa 1551.8 MPa Yield Strength @ 0.2%  378 MPa 1197.6 MPa Offset Elongation 54.6% 21.8% Uniform Elongation 51.5% 20.4% Plastic Strain Ratio 0.80 0.86 LDH (Limiting Dome 2.26″ 1.32″ Height) LDR (Limiting Draw Ratio) 1.9 1.5 HER (Hole Expansion 5%, 10%, 21%, 3.6%, 11.2%, 20.6%, Ratio) (0.25 mm/s, 8 mm/s, 39%, 45% 20.1%, 23.2% 28 mm/s, 114 mm/s, 228 mm/s) Hardness 98 HRB 46.6 HRC

Example 4

The effect of warm rolling on anistropy was also studied on the metastable steel of Example 1. Anisotropy can have a significant effect on subsequent forming. Warm rolling helped manage anisotropy in mechanical properties of metastable steels.

The effect of warm rolling compared to cold rolling is further demonstrated by the data set forth in Table 3 below. The initial hot band was the same for both sets of rolling. One set was warm rolled (@˜250 F) to various reductions (10, 15 and 20%) , the other was cold rolled to similar reductions. In the case of the cold rolled samples, elongations in longitudinal (L) and transverse (T) orientations differ quite a bit. The higher the amount of reduction, the bigger is the said difference. However, in the case of warm rolling, the difference is much smaller.

TABLE 3 Cold Rolling Warm Rolling % % % Reduction L T Difference L T Difference % Elongation 10.0 21.1 15.0 40.7 22.1 16.4 34.8 15.0 18.8 10.1 86.1 19.7 12.6 56.3 20.0 18.1 7.6 138.2 19.8 13.6 45.6 UTS 10.0 1114.0 1073.7 3.8 1065.4 1059.7 0.5 15.0 1226.6 1161.7 5.6 1199.3 1147.0 4.6 20.0 1321.2 1204.3 9.7 1226.2 1190.5 3.0 0.2% OYS 10.0 577.8 600.2 −3.7 534.5 596.7 −10.4 15.0 724.9 688.8 5.2 664.3 674.1 −1.5 20.0 789.3 719.2 9.7 736.1 689.3 6.8 

What is claimed is:
 1. A method of rolling a metastable steel comprising the steps of: a. Selecting a metastable steel having an instability factor (IF) greater than or equal to 2.9, wherein IF is calculated by the following equation: IF=37.193-51.248(% C)−0.4677(% Cr)−1.0174(% Mn)−34.396 (% N)−2.5884(% Ni) b. Prior to rolling, warming said metastable steel to a warming temperature greater than 80° F.; and c. Rolling said metastable steel.
 2. The method of claim 1, wherein the warming temperature is near or above the M_(d) temperature for the particular metastable steel composition.
 3. The method of claim 1, wherein the warming temperature is near or above the M_(d)30 temperature for the particular metastable steel composition.
 4. The method of claim 1, wherein the warming temperature is less than or equal to 250° F.
 5. The method of claim 3, wherein the M_(d)30 temperature for the metastable steel is calculated according to the following equation: M _(d)30=551−462(% C+% N)−68*% Cb−13.7*Cr−29(% Cu+% Ni)−8.1*% Mn−18.5*% Mo−9.2*% Si
 6. The method of claim 3, wherein the M_(d)30 temperature for the metastable steel is calculated according to the following equation: M _(d)30=413-462*(% C+% N)−13.7*% Cr−8.1*% Mn−18.5*% Mo−9.5*% N %−9.2*% Si
 7. The method of claim 1, 2, 3, 4, 5, or 6, further comprising the step of wherein after rolling, the metastable steel is rolled at room temperature.
 8. The method of claim 7, further comprising the step of wherein the metastable steel is annealed prior to being rolled at room temperature.
 9. The method of claim 1, 2, 3, 4, 5, or 6 further comprising the step of wherein after rolling, the metastable steel is rolled in a second rolling step and prior to the second rolling step, the metastable steel is warmed to a warming temperature greater than 80° F. 